Tasks concerning the problem of the aging of the social infrastructures developed in the high growth period are to adopt appropriate maintenance schemes to achieve longer life and to reduce and level the total cost of maintenance and renewal. To accomplish the tasks, it is expected to promote the development of monitoring technology that makes it possible to efficiently grasp the states of social infrastructures by utilizing sensors, IT, and the like.
The measurement modes of the monitoring technology include a fixed type for constant monitoring and a mobile type for regular monitoring. For the fixed type, sensors, cameras, or the like are installed in a structure (a concrete structure, a steel structure, an earth structure, a ground, or the like). For the mobile type, a movable body (a vehicle or the like) on which a sensor, a camera, or the like is installed is used. The currently predominant measurement mode is the fixed type. A typical example of the fixed type may include monitoring the secular change in the amount of strain with a strain sensor installed on a concrete structure (a tunnel wall surface, a road surface, or the like) and a steel structure (a bridge or the like).
As a strain sensor used for such a fixed monitoring technique, for example, JP-A-2009-79976 discloses a sensor that measures, at multiple points, strains of a road surface on which traveling wheels of a moving object are in contact. Specifically, in this strain sensor 1 that enables multipoint measurement, a gauge base 4, to which a plurality of resistance type strain gauges 3 is laminated, is laminated onto the back surface of a flexible substrate 2. Further, a sheet-shaped insulating member (cover film) 5 is laminated onto the surface of the flexible substrate 2, thereby providing a sensor structure part 6 (see FIGS. 13 and 14). A thin film conductor (routing circuit) 18 and thin film conductors (routing circuits) 19, 20 are provided on the flexible substrate 2. The thin film conductor (routing circuit) 18 conducts to a lead connection part 14a of lead connection parts (tabs) 14a, 14b at both ends of a strain-sensing part 13 of each resistance type strain gauge 3. The thin film conductors (routing circuits) 19, 20 conduct to the lead connection part 14b. A gauge base 4 of the sensor part 6 is installed on the road surface in such a way that the strain gauge 3 is located in the area of the road surface that the wheels contact, and ends 18b, 19b, 20b of the thin film conductors 18, 19, 20 are located in an area where the ends deviate from the contact area in the width direction of the wheels.
Various embodiments are disclosed in JP-A-2009-79976. For example, the flexible substrate 2 shown in FIGS. 13 and 14 has a two-layer structure composed of a first layer substrate 7 and a second layer substrate 8. Alternatively, a single layer flexible substrate may be used. Further, in FIG. 14, a plurality of strain gauges 3 having a gauge base 12 and a strain-sensing part 13 are used. Furthermore, these strain gauges 3 are laminated onto the surface of the gauge base 4. Alternatively, a plurality of strain-sensing parts 13 may be directly formed on the surface of the gauge base 4 by photo etching or the like. Further, in FIGS. 13 and 14, gauge leads 15a, 15b are respectively connected to the tabs 14a, 14b at both ends of each strain-sensing part 13. Thus, conduction between the tab 14a and the thin film conductor 18 and conduction between the tab 14b and the thin film conductors 19, 20 are carried out via the gauge leads 15a, 15b. Alternatively, it is also possible to provide conduction between the tab 14a and the thin film conductor 18 and conduction between the tab 14b and the thin film conductors 19, 20 without using the gauge leads 15a, 15b. 